SCD is a serious congenital disease which affects 1 in 500 African-Americans, as well as individuals of other racial backgrounds, exacting a substantial toll in morbidity and mortality upon the approximately 100,000 Americans afflicted. SCD patients can have frequent episodes of severe, debilitating pain, often requiring emergency room visits or hospitalization, and time off from work or school. Although some patients respond to hydroxyurea (HU, the standard treatment for symptomatic patients), and a few may be candidates for allogeneic stem cell transplantation, most patients continue to suffer. Furthermore, HU is used at doses that cause acute DNA damage and cytotoxicity, and has potential genotoxic, teratogenic and anti-fertility effects. Similarly, allogeneic stem cell transplantation is performed with cytotoxic conditioning and attendant risks of treatment-related mortality.
A clinical research effort to develop pharmacologic inducers of HbF expression culminated in FDA approval of the anti-metabolite HU to treat symptomatic SCD in 1998. A recent follow-up of patients enrolled in the pivotal HU trial confirmed that a decreased risk of mortality in these patients correlates with HbF levels (Steinberg, M. H., et al. 2003 JAMA 289: 1645-1651; Rosse, W. F., et al. 2000 Am Soc Hematol Educ Program 2-17). However, HbF levels are not increased in approximately 40% of HU compliant patients (Steiberg, M. H., et al. 1997 Blood 89: 1078-1088; Steinberg, M. H., et al. 1999 Expert Opin Investig Drugs 8:1823-1836; Atweh, G. F., et al. 2001 Curr Opin Hematol 8:123-130). Finally, HU is used at DNA-damaging, cytotoxic doses, potentially compounding the bone marrow damage that accumulates in SCD.
Most cells in the body, including cancer cells or blood disorder cells, such as sickle cells, contain the same complement of genes. The function and specialization of a cell is, therefore, determined by which of these genes are turned-on (activated), and which are turned-off (repressed). Activation refers to the expression of the protein encoded by the gene, while repression of the gene implies that the protein encoded by that gene is expressed at lower levels or not at all. DNA methyl-transferase 1 (DNMT1) is an enzyme which plays a critical and central role in the machinery that represses genes. Therefore, altering the levels of DNMT1 within a cell can have powerful effects on the pattern of gene-expression, function and specialization of a cell.
Decitabine (5-aza-2′-deoxycytidine) is a nucleoside analogue drug—a drug that mimics a natural component of DNA. Decitabine is relatively unique amongst the large family of nucleoside analogue drugs in that it can irreversibly bind to and deplete DNMT1.
Cytidine deaminase (CAD) is an enzyme that is highly expressed in the liver and intestine and rapidly destroys decitabine within the body. Tetrahydrouridine (THU) is a safe and well-tolerated pyrimidine nucleoside analogue that inhibits cytidine deaminase. In humans, the cytidine deaminase gene is subject to non-synonymous single nucleotide polymorphisms which produce variants of cytidine deaminase that have differences in enzymatic activity of 3-fold or more (Gilbert, J. A., et al. 2006 Clin Cancer Res 12, 1794-1803; Kirch, H. C., et al. 1998 Exp Hematol 26, 421-425; Yue, L., et al. 2003 Pharmacogenetics 13, 29-38).